Since 1975, the United States experienced remarkable growth in the usage of home smoke detectors, principally single-station, battery-operated, ionization-mode smoke detectors. This rapid growth, coupled with clear evidence in actual fires and fire statistics of the lifesaving effectiveness of detectors, made the home smoke detector the fire safety success story of the past two decades.
In recent years, however, studies of the operational status of smoke detectors in homes revealed an alarming statistic that as many as one-fourth to one-third of smoke detectors are nonoperational at any one time. Over half of the nonoperational smoke detectors are attributable to missing batteries. The rest is due to dead batteries and nonworking smoke detectors. Research showed the principal cause of the missing batteries was homeowner's frustration over nuisance alarms, which are caused not by hostile, unwanted fires but by controlled fires, such as cooking flames. These nuisance or false alarms are also caused by nonfire sources, such as moisture emanating from a bathroom after someone has taken a shower, dust or debris stirred up during cleaning, or oil vapors escaping from the kitchen.
Centralized fire detection systems also play an important role in protecting the occupants of commercial and industrial buildings. False alarms are detrimental in this setting as well, not only causing inconvenience to building occupants but also creating a dangerous lack of confidence in the validity of future alarms.
The reason the majority of smoke detectors, which are of the ionization type, are prone to these types of nuisance alarms is that they are very sensitive to visible and invisible diffused particulate matter, especially when the fire alarm threshold is set very low to meet the mandated response time for ANSI/UL 268 certification for various types of fires. Visible particulate matter ranges in size from 4 to 5 microns in a minimum dimension (although small particles can be seen as a haze when present in high mass density) and is generated copiously in most open fires or flames. However, ionization detectors are most sensitive to invisible particles ranging from 1.0 to 0.01 micron in a minimum dimension. Most household nonfire sources, as discussed briefly above, generate mostly invisible particulate matters. This explains why most home smoke detectors encounter so many nuisance alarms.
The problem of frequent false alarms, caused by ionization smoke detectors and resulting in a significant portion of them at any one time being functionally nonoperational, led to the increased use in recent years of another type of smoke detector, namely the photoelectric smoke detector. Photoelectric smoke detectors work best for visible particulate matter and are relatively insensitive to invisible particulate matter. They are therefore less prone to nuisance alarms. However, the drawback is that they are slow in responding to flaming fires in which the early particulate matter generated is mostly invisible. To overcome this drawback, the fire alarm threshold of photoelectric smoke detectors has to be set very low or at high sensitivity to meet the ANSI/UL 268 certification requirements. Such low fire alarm thresholds for the photoelectric smoke detectors also lead to frequent false alarms. Thus the problem of nuisance false alarms for smoke detectors comes full circle. It is apparent that over the years the problem has been long recognized but has not been solved. It is equally apparent that a new type of fire detector is urgently needed to resolve the dangerous ineffectiveness of present-day smoke detectors.
Another aspect of present-day smoke detectors that is often discussed but seldom addressed is the slow fire response of these detectors. The current ANSI/UL 268 fire detector certification code was developed and dictated years ago by the fire detection technology, viz., the technology of smoke detectors. Opinion of workers in the fire fighting and prevention industry over the past two decades has always been critical of the speed of response of the available smoke detectors. Obviously, increasing their sensitivity through the lowering of the light obscuration detection threshold of the smoke detectors will speed up their response. However, lowering the detection threshold also drives up the nuisance alarm rates. Looking from this perspective, it is also apparent that a better fire detector is urgently needed.
Taking advantage of the copious production of CO.sub.2 gas by virtually all manner of fires, a new type of fire detector keying on the detection of CO.sub.2 gas was disclosed by Jacob Y. Wong, one of the present inventors, in U.S. Pat. No. 5,053,754. This new fire detector responds more rapidly to fires than the widely used smoke detectors. It senses increases in the concentration of CO.sub.2 associated with a fire by measuring the concomitant increase in the absorption of a beam of radiation whose wavelength is located at a strong absorption band of CO.sub.2. The disclosed device is considerably simplified by the use of a sample chamber window that is highly permeable to CO.sub.2 but which keeps out particles of dust, smoke, oil, and water.
In subsequent U.S. Pat. Nos. 5,079,422; 5,103,096; and 5,369,397, inventor Wong continued to disclose a number of improved methods of using single or multiple CO.sub.2 detectors to detect fires. The superiority of using CO.sub.2 detectors as fire detectors over smoke detectors in terms of speed of response and immunity against common nuisance alarms has been well established. In co-pending patent application No. 08/077,488, filed Nov. 14, 1994, for FALSE ALARM RESISTANT FIRE DETECTOR WITH IMPROVED PERFORMANCE and U.S. patent application Ser. No. 08/593,253, filed Jan. 30, 1996, for AN IMPROVED FIRE DETECTOR, inventor Wong further disclosed the advantage of combining a CO.sub.2 detector with a smoke detector to form a fast and false alarm-resistant fire detector.
Even though advantages of using CO.sub.2 detectors as fire detectors have been proposed, the reality is that until such time as the manufacturing cost of a nondispersive infrared ("NDIR") CO.sub.2 detector is reduced to an economically attractive level, the consumer will be unwilling to purchase this new and improved fire detector because of hard-nosed economics. The concomitant effort to simplify and reduce the cost of an NDIR CO.sub.2 detector is therefore equally important and relevant in forging the advent of the currently disclosed practical and improved fire detector.
In U.S. Pat. No. 5,026,992, inventor Wong disclosed a novel simplification of an NDIR gas detector with the ultimate goal of reducing the cost of this device to the point that it can be used to detect CO.sub.2 gas in its application as a new fire detector as discussed above. U.S. Pat. No. 5,026,992 disclosed a spectral ratio forming technique for NDIR gas analysis using a differential temperature source that leads to an extremely simple NDIR gas detector comprising only one infrared source and one infrared detector. In U.S. Pat. No. 5,163,332, inventor Wong disclosed the use of a diffusion-type gas sample chamber in the construction of an NDIR gas detector that eliminated virtually all the delicate and expensive optical and mechanical components of a conventional NDIR gas detector. In U.S. Pat. No. 5,341,214, inventor Wong expanded the novel idea of a diffusion-type sample chamber of U.S. Pat. No. 5,163,332 to include the conventional spectral ratio forming technique in NDIR gas analysis. In U.S. Pat. No. 5,340,986, inventor Wong extended the disclosure of a diffusion-type gas chamber in U.S. Pat. No. 5,163,332 to a "re-entrant" configuration, thus simplifying even further the construct of an NDIR gas detector. Still further simplification is required if CO.sub.2 sensors are to gain acceptance in low-cost household fire detectors and thus fulfill the long-felt need for an improved fire detector with a lower response time that still minimizes false alarms.
There are also problems caused by the fact that modern buildings, such as office buildings, typically include both a fire control system and an air conditioning system. The fire control system typically includes numerous fire detectors that measure a condition, such as smoke density, at locations throughout the building. In the event a fire is detected, an alarm is typically sounded. Unfortunately, the air conditioning system frequently works to both prevent detection of a fire and to facilitate the growth of a fire. This happens when the air-conditioning system blows air into the area, diluting the concentration of smoke and fire by-product gasses and thereby delaying or preventing detection. This air also supplies new oxygen to the fire, facilitating its growth.
Alarm systems also typically issue a single type alarm signal only. There are, however, many types of fires with different levels of urgency. Although it might seem desirable to meet every fire alarm with the full capability of the local fire department, this would make the local fire department frequently unprepared for other fires. In the worst situation several fire trucks would respond to an alarm from a very slow burning fire and then be unable to reach a rapidly burning fire in a timely manner.
Heretofore, firefighters reaching a burning building entered the building with only scant information concerning the location of the flames and smoke. This placed the firefighters in considerable peril as they entered and explored the building, searching for flames while attempting to avoid thick smoke. Unexpectedly encountered flames and thick smoke have caused the death of many fire fighters.
There exist fire detection systems that are adapted for placement in air conditioning ducts. When a fire is detected, typically through the detection of smoke, this type of fire detector causes the air conditioning vent to shut, thereby helping to isolate the fire. This type of detector cannot, however, distinguish between a fire present within the vent or a fire in the building outside of the vent. This lack of information hinders detection of the fire.
In addition, heretofore available systems did not close the vent or vents until a fire indicating criterion was satisfied. The air conditioning system might already have been operating for a while at that point, both feeding the fire with oxygen and delaying the detection of the fire.
Yet another aspect of present day smoke detector systems is the expensive requirement for periodic maintenance. Atmospheric dust gradually accumulates on light emitting and receiving surfaces inside the smoke detectors, degrading their performance. Some smoke detectors can signal when the dust accumulation has degraded performance below a minimum acceptable level. These must be cleaned when indicated by the signal. Others must be cleaned according to a schedule designed to maintain performance above the minimum acceptable level.
In either case, the task of cleaning is nontrivial. Typically, the smoke detector must be removed from its location and replaced with a similar unit. The actual cleaning is performed at a separate location and involves a fair degree of labor and partial disassembly of the unit. Therefore a detector that reduces or eliminates the need for cleaning is highly desirable.